Wireless power: matching requirements with available technology

The promise of wireless power, or
wireless energy transfer, is certainly not new. Famously, Nikola Tesla
experimented with long-range wireless energy transfer, but his most ambitious
attempt — the Wardenclyffe Tower — failed when he ran out of money. Since
then, scientists and engineers have made great strides in advancing the
technology, but many feel that the technology still falls short of its lofty
expectations.

The reality of wireless power is
that we are not as far away as some might think. Rather, it’s a matter of
understanding the currently available technology and matching that technology
with appropriate use cases.

Our wireless worldWireless power technologies
deliver electricity to charge and power devices without being plugged in. For
example, a smart speaker could be placed anywhere in the home without routing
ugly cords. Currently, most “wireless” objects rely on batteries that
eventually need to be charged or replaced.

These battery-operated devices
often trade performance and functionality for battery life. A battery-operated
security camera might be able to stream just one minute of video per day. With
wireless power, that limitation is removed.

So which devices can be powered
without wires? That depends. Different devices have different power needs. Don’t
expect to disconnect your refrigerator or TV from the power outlet any time
soon. But if you want to charge a phone across the room and have wire-free
surround speakers, smart home devices, or game controllers, the right wireless
power technology can make this a reality.

Two sides of the spectrumTo better understand our options
for wireless power, we need to look at the entire electromagnetic (EM)
frequency spectrum. The EM frequency bands are shown in Fig. 1 below.

Fig. 1: The electromagnetic spectrum

Of primary interest are the
commercially developed bands of radio and infrared (IR). Between them — at the
range of 100 GHz to 10 THz — is a technological chasm called the “THz gap,” where
little is available in terms of commercial components. Little is also known
about the health effects of that “THz gap” band.

Visible light and ultraviolet
light aren’t great candidates for wireless power due to inherent user
experience and safety challenges. Thus, when we look at the EM spectrum, we are
essentially comparing the RF and IR bands, which are on two sides of the EM
spectrum.

The power playHow much power do you need and
how much can you deliver? Power is measured in watts (W). Obviously, different
devices will require different amounts of power to operate or charge. Included
below are typical power consumption values for several devices.

Understanding that different
devices have different needs is an important first step in determining which
devices are better suited for wireless power as it exists today. But that’s
just the first part of the question. We also need to look at how much power can
be delivered. To do that, we need to look at the main factors that limit energy
delivery: distance (limited by diffraction) and power (limited by safety).

Diffraction: Imagine someone
trying to get a drink from a garden hose. The water is “transmitted” from the
garden hose and “received” by the person’s mouth. The water stream coming out
of the garden hose has a small diameter. The farther away the person is from
the garden hose, the wider the diameter of the water stream, meaning that a smaller
percentage of the available water will be captured as they move farther away
from the hose. More water will be “wasted” or will drench other people or
objects.

In a similar fashion, when a
power beam “diverges” (becomes wider), the distance of the transmitter from the
receiver impacts the power-capture ability. If only a small portion of
transmitted energy is captured by the receiver, much more energy needs to be
sent from the transmitter to power the device.

One option to improve efficiency would
be to increase the receiver size, but creating a larger receiver makes the
technology less practical for phones or other portable devices.

Safety: If only a small portion of transmitted energy
is captured, then the rest of the power is radiated into the environment. Power
not captured by the receiver would expose people, animals, and objects to
collateral radiation. Depending on the amount and type of energy, this could be
unsafe.

Finding the right technologyIf you asked a group of engineers
to come up with a solution for long-range wireless power, radio frequency (RF)
would likely be a natural choice. There are many commoditized components
available for RF, and there is a lot of RF engineering talent. It seems logical
that if RF does so well in communications, it may also do well with wireless
power.

Unfortunately, it turns out that
the very properties that make RF so well-adapted to communication cripple it
when it comes to wireless power delivery. When you try to pack RF into a tight
beam and send it over to some distant point, it tends to disintegrate very
quickly. In fact, it turns out that delivering RF beams over distances beyond 1
m is nearly impossible. We’re talking about practical implementations for
electronic devices, not huge antennas and radio-telescope dishes.

Similarly, when you crank up the
power level, you discover that going beyond 0.1 W or so violates the applicable
safety standards for radio. Eventually, you end up with a modest envelope of
potential performance (much greater than magnetic induction), but again, you
are limited by reach and the power level that you can support.

RF may be able to marginally
support some smart home applications. For example, it can support peripherals
like wireless mice or keyboards in a desktop environment — applications that
don’t require anything beyond 1 m. But achieving meaningful support of the vast
majority of smart home applications and personal mobile devices remains outside
of the realm of RF technology.

It turns out that if you look for
a technology that can power both mobile devices and smart devices anywhere in
the room at distances of 10 m and power levels of up to 10 W, IR is a good
choice because it has two magnificent properties that make it well-suited for
delivering wireless power.

First, you can pack a very tight
beam of infrared power and carry it over long distances. Think of a laser pointer
that can emit a focused beam of light over a long distance without
disintegrating. With IR, we have the capability to cover long distances without
losing shape or disintegrating and still deliver all of the power to the client
device.

The second property of IR, which
is even more attractive, relates to safety. RF is man-made radiation that was basically
invented about 100 to 150 years ago. RF can be quite harmful to all forms of
life, which is why safety limits kick in at such an early point. IR, on the
other hand, has always been around — it makes up about half of the solar energy
heating the earth. When life on Earth developed, it developed bathing in IR. The
result is that we are much more adapted to IR and that IR exhibits roughly 100× more
relaxed safety limits than radio.

If you combine the physical
phenomena of dispersion and the safety limits, you get the following
performance envelope for the available technologies:

Fig. 2: Power options

We plotted this in the context of
the customer experience (charging on a pad, on a desk, in a room, and beyond)
and power levels required for peripherals, smart home devices, and smartphones.

Conclusion
Although we may still be a few years away from powering large appliances
like laptops via wireless power, the combination of efficiency and safety show
that IR can deliver 100× the energy of best-case RF technologies. In other words, IR technology can
deliver sufficient power to charge most modern mobile phones.